The present invention relates to a fluorine-containing polyolefin non-woven fabric and a method of manufacturing such fabric. The present invention also relates to a separator for a cell using the fabric of the type described.
Commonly available non-woven fabrics are made of fibers including natural and synthetic ones. The individual fibers have advantages as well as disadvantages. For example, non-woven fabrics of nylon fibers are superior in hydrophilic nature, dying properties, and printability. These fibers are, however, inferior in chemical resistance, solvent resistance, and weathering resistance. On the contrary, non-woven fabrics of polyolefin fibers are superior in the chemical resistance, the solvent resistance, and the weathering resistance, which are disadvantageously inferior in the hydrophilic nature, the dying properties, and the printability. Accordingly, development of a non-woven fabric has been desired that is superior in the chemical resistance, the solvent resistance, and the weathering resistance and has good hydrophilic nature, dying properties, and printability as well.
One application of the conventional non-woven fabric is a separator for a cell. Typical separators may be a nylon non-woven fabric or a polyolefin porous film. However, when used as the separator for a nickel-hydrogen cell or a nickel-zinc cell using an alkali solution as the electrolyte, the fabric should be highly hydrophilic and have superior alkali resistance.
For the conventional separator of this type has been used a polyolefin non-woven fabric which is sulfonated to impart the hydrophilic nature. However, this sulfonated polyolefin non-woven fabric is insufficient in view of the alkali resistance. Alternatively, the polyolefin non-woven fabric may be modified by treatment with other materials. For example, Japanese Patent Publication No. 59-5601 discloses to impart the hydrophilic nature to the polyolefin non-woven fabric by means of surface treatment with a mixture of fluorine gas and oxygen gas. Japanese Patent Publication Nos. 4-7548 and 5-46056 disclose the use of the non-woven fabric so treated as a separator for a cell. These fabrics are, however, also insufficient in view of the alkali resistance.
With these respects, the present inventors have been made tremendous studies to overcome the above mentioned problems. As a result, it has been found that a fluorine-containing polyolefin non-woven fabric where the numbers of carbon, oxygen, and fluorine atoms of a surface elemental composition are in a certain relation is superior in the chemical resistance, the solvent resistance, and the weathering resistance and has good hydrophilic nature, dying properties, and printability as well. It has also been found that the fluorine-containing polyolefin non-woven fabric of the type described has the superior alkali resistance. The present invention was thus completed.